Why did protons appear when alpha particles struck nitrogen in Rutherford’s experiments?

In 1919, Ernest Rutherford discovered that protons appeared when alpha particles struck nitrogen because he had successfully induced the first artificial nuclear reaction in a stable element.21

When the energetic alpha particles collided with the nitrogen nuclei, the interaction caused a change in the identity of the target nucleus. Specifically, the nitrogen nucleus absorbed the alpha particle and ejected a lighter particle, which Rutherford identified as a hydrogen nucleus, or proton. This process transformed the nitrogen into a rare oxygen isotope.1

This experiment was significant because it demonstrated that the nucleus of a stable element could be altered through bombardment, marking a foundational moment in the development of nuclear physics.2